1. Field of the Invention
The present invention relates to an image forming method using a photothermographic material, particularly to an image forming method by exposing a photothermographic material including a silver halide emulsion having a high silver iodide content to light with an wavelength of 350 nm to 450 nm. The invention also relates to an image forming method for a photothermographic material which provides improvements with respect to uneven image density in a sub-scanning direction by suppressing generation of interference fringes.
2. Description of the Related Art
In the medical imaging field and the graphic arts field, there has been, in recent years, a strong desire for a dry photographic process from the viewpoints of environmental conservation and space-saving. Further the development of digitization has resulted in the rapid development of systems in which image information is captured and stored into a computer. If necessary, the image information is processed by the computer which outputs the image information through communication to a location where the image formation is needed. At the site, the image information is further output onto a photosensitive material using a laser-image setter or a laser-imager, followed by development thereof to form an image on the photosensitive material. It is required that the photosensitive material be able to record an image under exposure to laser with a high intensity, and that a clear black-tone image with a high resolution and sharpness can be formed. While various kinds of hard copy systems using a pigment and a dye such as an ink-jet printer or electrophotography have been distributed as a general image forming system using such a digital recording imaging material, images in the digital recording imaging material obtained by such a general image forming system are insufficient in terms of image qualities required for medical images. To facilitate diagnosis, image qualities such as sharpness, granularity, gradation, tone and high recording speed (sensitivity) are required. However, digital recording imaging material has not reached a level at which it can replace medical silver salt film processed by conventional wet development.
A thermographic system using an organic silver salt is described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075; and D. Klosterboer, “Thermally Processed Silver Systems”, in “Imaging Processes and Materials” coedited by J. Sturge, V. Walworth and A. Shepp, Neblette 8th Ed., Chapter 9, pp. 279–291, 1989, the disclosure of which is incorporated by reference herein. Generally, a photothermographic material, in particular, comprises an image forming layer in which a photosensitive silver halide, a reducing agent, a reducible silver salt (for example, an organic silver salt) and if necessary, a toner controlling a color tone of diveloped silver are dispersed in a binder matrix.
A black-toned silver image is formed by heating a photothermographic material to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by a catalytic action of a latent image generated on the silver halide by exposure. As a result, a black-toned silver image is formed in an exposed region. Such a photothermographic material is disclosed in the literature including for examples U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924. Fuji medical dry imager FM-DP L is an example of a practical medical image forming system that has been marketed.
Since such a photothermographic system using an organic silver salt has no fixing step and the photosensitive material contains all chemicals necessary for image forming, there has been an intrinsic problem in raw preservability, that is, “increase in fog”, in which an unexposed portion is blackened during storage from manufacture of a photosensitive material until the material is actually put into use, and another intrinsic problem of “print-out”. In “print-out”, an image after thermal development is exposed to illumination of weak light such as room light, and an unexposed portion is gradually blackened.
As means for reducing fogging and print-out, a method in which silver iodide formed through conversion of an organic silver salt is used is disclosed in U.S. Pat. No. 6,143,488, EP No. 0922995, the disclosures of which are incorporated herein by reference. Other photosensitive materials using silver iodide are disclosed in WO Nos. 97-48014 and 97-48015; U.S. Pat. No. 6,165,705; JP-A No. 8-297345; and Japanese Patent No. 2785129.
While such a photothermographic material using a silver halide with a high silver iodide content is a remarkable material in its ability for preventing the generation of fogging and print out, there has been a problem in that the photothermographic material is generally low in sensitivity.
On the one hand, exposure light sources preferably used in recent years have been red to infrared laser diodes having a peak laser beam wavelength of 600 nm to 900 nm. A peculiar problem of interference fringe patterns appearing in a formed image in the image forming method by scanning the red to infrared laser diode beam should be solved. The interference fringe of this sort is thought to arise from interference between light incident onto the surface of an image forming layer and transmission light, which is not absorbed in the image forming layer of the photothermographic material, and is reflected multiple times in the image forming layer including a support. Conventionally, a dye and pigment have been added in the image forming layer for absorbing the scattered light, and a light scattering pigment has been added, in order to prevent the interference fringes from being generated. However, these pigments and dyes may unfavorably color the layer, and turbidity of the layer may be increased.
On the other hand, an incidence angle of the laser beam may be tilted in order to prevent the interference fringes from being generated. However, the tilt angle of the laser beam should be larger for improving the interference fringe.
It is not preferable to largely increase the incidence angle of the laser beam due to the problem of ghosts, which appear due to surface reflection of the laser beam, as well as the problem of blurring of the image.
Since the laser beam can only be tilted within a limited range, the effect of improving the interference fringe has inevitably been restricted. Consequently, improvements have relied on addition of the dyes.
With respect to the blue laser diode as well, it is a crucial problem to prevent the interference fringe from being generated. Accordingly, being able to use a silver halide emulsion with a high content of silver iodide, which has a characteristic light absorption in the wavelength region of the blue laser diode, is an important issue for meeting future demands and is ardently desired.